The present invention relates to a method for forming a gel in an underground formation surrounding a wellbore for the purpose of decreasing permeability in the formation. More particularly, the invention concerns an in situ formed lignosulfonate and acrylic gel.
Poor vertical conformance and excessive water production are two problems which afflict most hydrocarbon producing formations. Because of the existence of different geological strata having varying permeabilities, fluids injected into and produced from hydrocarbon formations tend to preferentially channel into strata of relatively high permeability. The result is that enhanced oil recovery fluids bypass large concentrations of hydrocarbons and flow through strata that have already been swept of hydrocarbons. This also results in uneconomically high water to oil ratios from producing wells.
Numerous processes have been proposed to alleviate preferential channeling through formation strata of relatively high permeabilities. Most methods involve the injection of an externally formed or in situ formed gel to lower permeability. U.S. Pat. Nos. 4,412,028; 4,494,606; 4,498,539; 4,503,912; 4,561,502 and 4,579,667 all describe the injection of polyacrylamide gels into a formation for profile control. Some polyacrylamide gels suffer from a lack of stiffness. They soften and do not always stay where placed. Polyacrylamides also have limited stability under acid conditions.
Because of their structure and inexpensive cost, lignins and lignosulfonates have been frequently used to form permeability reducing gels. Reissue Patent No. 30,767 and U.S. Pat. No. 4,074,757 inject lignosulfonate and water in the absence of other gelation promoters. These gels are set by high formation temperatures. U.S. Pat. No. 3,896,827 describes a lignosulfonate gel formed by injecting a lignosulfonate solution with a mixed activator comprised of a dichromate and a alkali metal or alkaline earth salt. Lignosulfonate and silicate gels are described in U.S. Pat. Nos. 4,257,813 and 4,275,789. Lignosulfonate gels formed with a dichromate initiator have also been used to thermally insulate wellbores, such as disclosed in U.S. Pat. No. 4,296,814. U.S. Pat. No. 4,428,429 discloses the injection of a lignin solution which will gel within the formation. A profile controlled gel formed from lignosulfonate and carboxymethylcellulose is disclosed in U.S. Pat. No. 4,110,231.
Gels containing lignosulfonates and lignins are also used as viscosity enhancers for polymer flooding and aqueous drilling fluids. Enhanced Recovery Week of June 10, 1985 describes a viscosity enhancer for polymer flooding on page 5. In this method, a lignin-acrylamide graft copolymer is placed in aqueous solution to waterflood reservoirs. However, the researchers working with the lignin-acrylamide copolymer system have been unable to attain sufficiently high enough viscosity for an effective waterflooding system.
Lignosulfonate and polyacrylate copolymers have also been employed in aqueous drilling fluids to increase the viscosity and the ability of the drilling fluids to suspend and carry away drill cuttings. U.S. Pat. Nos. 3,985,659 and 4,322,301 employ lignosulfonate and polyacrylate copolymers to increase the thixotropic properties of the clay drilling fluids while maintaining fluidity. Both methods employ peroxide initiators to copolymerize the lignosulfonate and polyacrylate.